As a result of the rapid development of liquid crystal display technologies, liquid crystal display devices have been utilized in a wide variety of applications. Liquid crystal display devices require low voltage and low power of operations, both of these advantageous factors enable them to be the most promising of the non-emissive electro-optical display candidates. Traditionally, liquid crystal displays are used in devices that require compact, energy-efficient voltage-controlled light switches, such as watch and calculator displays. More recently, liquid crystal displays have been increasingly used in large area computer display screens, as well as flat screen TV's. Conventional liquid crystal display devices are based on the dielectric alignment effects in nematic liquid crystal compounds. By virtue of dielectric anisotropy, the average molecular long axis of the liquid crystal compound takes up a preferred orientation in an applied electric field. However, because the coupling to an applied electric field by this mechanism is relatively weak, the resultant electro-optical response time may be too slow for more complicated applications. The conventional liquid crystal displays also do not provide adequate or desired contrast and wide enough view-angle for many potential applications.
The existence of ferroelectric liquid crystals was first observed in 1974 which was subsequently published in 1975 by R. B. Meyer, L. Liebert, L. Strzelecki, and P. Keller (see J. Physique Letters, 1975, 36, L-69). They reported that strong ferroelectricity could be observed from liquid crystals belonging to a chiral smectic C phase (Sc* phase). Prior to the discovery by Meyer, ferroelectricity was observed only in inorganic salt crystals. Meyer and his coworkers believed that ferroelectricity may be observed in optically active compounds with a tilted smectic liquid crystal phase. They synthesized a liquid crystal compound (S)-4-n-decyloxybenzylideneamino-2'-methylbutyl cinnamate (DOBAMBC) to prove their theory. Therefore, in theory, any optically active compound that exhibits tilted smectic phase can possess ferroelectricity. Currently, the chiral smectic C-phase (Sc*) compounds are receiving the most attention.
In 1980, N. A. Clark and S. T. Lagerwall proposed a liquid crystal light valve system wherein an optical switching phenomenon of a ferroelectric liquid crystal was utilized. (See Appl. Phys. Lett., 1980, 36, 899; see, also, U.S. Pat. No. 4,367,924, entitled "Chiral Smectic C or H Liquid Crystal Electro-Optical Device"). The discovery of Clark and Lagerwall opened the door for ferroelectric liquid crystals to be used in practical applications. One of the improvements advanced from their discovery is the use of surface stabilized ferroelectric liquid crystal device (SSFLCD) as a operating mechanism.
Chiral smectic C phase (Sc*) compounds possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, derived from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phase. Upon an appropriate application of an external electric field, the molecules in the ferroelectric liquid crystal phase will be aligned with the applied field. When the sign of the applied field is reversed, a "switching" of the ferroelectric liquid crystal molecules occurs. This switch can be employed for light modulation. It has been reported in the art that, over a wide range of electric field strengths, the response time (.tau.) of a ferroelectric liquid crystal display is inversely proportional to the strength of the applied field (E) and the spontaneous polarization of the ferroelectric liquid crystal molecule (Ps), and is proportional to the rotational viscosity (.eta.), as described in the following relationship: EQU .tau.=.eta./(Ps.times.E) (1)
In Eqn. (1), .eta.is the viscosity of the ferroelectric liquid crystal, and E is the externally applied electrical voltage. From Eqn. (1), it is apparent that the response time of a ferroelectric liquid crystal can be reduced through appropriate molecular design to emphasize certain desired molecular characteristics. For example, a proper increase in the dipole moment of the ferroelectric molecule can increase the spontaneous polarization thereof and thus reduce the response time.
Ferroelectric liquid crystal materials are the key component of a ferroelectric liquid crystal display device. In practical applications, however, a single ferroelectric liquid crystal material may not provide all the desired properties for an intended ferroelectric liquid crystal display device. Therefore, it has been disclosed that a chiral liquid crystal compound can be used as a dopant to mix with a liquid crystal host material, which may or may not be composed of chiral molecules, to form a ferroelectric liquid crystal mixture. Addition of the dopant can affect the spontaneous polarization and the viscosity of the chiral smectic phase, and thereby directly affect the response time as indicated by Eqn. (1).
U.S. Pat. No. 4,923,633 discloses a chiral liquid crystal dopant composition for use in a ferroelectric smectic liquid crystal composition having a structure which contains two chiral centers from alpha hydroxy carboxylic acid groups. U.S. Pat. No. 5,130,048 discloses another chiral liquid crystal dopant composition containing a pair of 2,4-dihalobutanediol chiral units. Both compounds involve relatively complicated synthesis procedures, and they do not provide adequate increase in the spontaneous polarization of the ferroelectric liquid crystal materials.